1. Field of the Invention
This invention pertains to the field of stepper motor position control. More particularly, the invention is a method and apparatus for controlling the power to a stepper motor based on stepper motor position information feedback. By way of further characterization, but not by way of limitation thereto, the invention is a method and apparatus which combines position information feedback with phase detection of the stepper motor windings to ensure precise stepper motor positioning.
2. Description of the Prior Art
In diagnostic instrumentation and other devices precise positioning of one element with respect to another is often required. For example, in some instruments a turntable containing numerous receptacles must be positioned at a precise point at allow access by probes or other devices in order that testing or other operations may be performed on the receptacle contents. The positioning accuracy required in such devices has been difficult to achieve.
Prior devices have employed servo systems including a DC motor drive with an optical sensor to detect the exact center of each receptacle. While suited for their intended purpose, these DC or other motor systems do not have the positioning accuracy or repeatability of a stepper motor system. However, while stepper motor systems are preferably from an engineering standpoint, their cost, when combined with the cost of optical feedback sensors, is greater than that for the servo systems. The required optical feedback sensors for stepper motors are expensive because the width of the mark on the turntable must be very precise and the optical detector must be very sensitive to give the required resolution.
A simpler and economically feasible solution would be to employ wider marks on the turntable and use less sensitive detectors. However, with this arrangement, the optical sensor may not "see" the mark at precisely the same position each time thus decreasing accuracy. In order to utilize this economically feasible arrangement an additional element is required to precisely position the stepper motor.